Вариант ...
<select>
<option value="bla1">bla1</option>
<option value="bla2">bla2</option>
<option value="bla3">bla3</option>
<option value="bla4">bla4</option>
<option value="bla5">bla5</option>
</select>
<script type="text/javascript">
function test(val){
var select = document.getElementsByTagName('select')[0];
var old = select.value;
select.value = val;
var rez = (select.value===val);
select.value=old
alert(val + "==" +rez)}
test("bla4")
test("bla7")
</script>